1. Field of the Invention
The present invention relates to surface design of vehicles, and particularly to the surface design of hypersonic vehicles.
2. Description of the Related Art
The transition of boundary-layer flows over hypersonic vehicle surfaces significantly affects the performance of hypersonic transportation vehicles and re-entry vehicles and the design of their thermal protection systems. The boundary-layer transition can have a first-order impact on lift, drag, control, and heat transfer properties of the vehicles. The ability to control the onset of transition to maintain laminar boundary-layer flows by delaying transition can result in lower drag, lower heat flux to surface, and higher fuel efficiency in hypersonic vehicles.
U.S. Patent Application Publication No. 2008/0265100 A1, entitled “Laminar Flow Surfaces with Selected Roughness Distributions, and Associated Methods,” is hereby incorporated by reference, and discloses a representative method for designing airfoils economically. The method enables the design of sweep wing beyond the maximum sweep angle typically associated with natural laminar flow wings, but without adding powered devices (cooling and/or blowing devices), by controlling surface roughness so that the disturbance amplification due to steady cross-flow instabilities is small enough to maintain laminar flow. According to this method, the maximum of the allowable surface roughness value is obtained. The associated manufacturing cost of wing surface is lowered because surface roughness does not need to be kept at very low value for the whole surface. Strictly speaking, this method is not for laminar flow control. It is more focus on lowering the manufacturing cost of sweep wings. The details of the effect of surface roughness on flow instabilities are not considered.
U.S. Pat. No. 6,892,989 B1, entitled “Method for Reducing The Drag of Blunt-Based Vehicles by Adaptively Increasing Forebody Roughness,” is hereby incorporated by reference, and provides a method for reducing total drag upon a blunt-based vehicle by adaptively increasing forebody roughness. Although the roughness leads to drag increase in the roughened area of the forebody, it significantly decreases drag at the base of the vehicle. As a result, total drag upon the vehicle decreases. It is quite clear that roughness here is used to trip flow transition.
U.S. Patent Application Publication No. 2004/0195462 A1, entitled “Surface Plasma Discharge for Controlling Leading Edge Contamination and Crossflow Instabilities for Laminar Flow,” is hereby incorporated by reference, and provides a method to delay transition associated with the leading edge contamination and crossflow instabilities by a complex closed-loop feedback control system. In this method, the control of the leading edge contamination and crossflow instabilities is achieved by changing Reynolds number of the supersonic boundary layer with volumetric heating introduced to the flow by plasma discharges. However, it is difficult to use such a complex system for a hypersonic boundary layer.
U.S. Pat. No. 5,884,871, entitled “Use of Absorbing Walls for Laminar Flow Control,” is hereby incorporated by reference, and provides a method for hypersonic laminar flow control by using ultrasonically transparent coating on the body surface. The pore size needs to be much less than the boundary-layer thickness so that the coating roughness will not trip the boundary layer. The porous coating absorbs energy from the second mode hence stabilizes flow. However, porous coating generally has no effect on the mean flow and the instability of the second mode, which make it difficult to achieve the efficient stabilization of hypersonic flows. The manufacturing of porous coating and the attachment of porous coating to body surface are not so convenient.
Employing a porous coating as described in U.S. Pat. No. 5,884,871 generally has no effect on the mean flow and the stability characteristics of the laminar flow. It stabilizes laminar flow by absorbing energy for the second mode. It is sometimes difficult for a porous coating to achieve efficient stabilization of hypersonic flows. For example, a porous coating destabilizes the first mode when it is located upstream of the synchronization point. In addition, the stabilization of porous coating depends on material properties of the coating and covering the vehicle surface with porous coatings may affect the strength and aerodynamic performance of the vehicle. Finally, it may be very difficult to put several porous coatings on a vehicle surface at different locations and removal of porous coatings from vehicle surface is difficult as well.
U.S. Pat. No. 5,618,363, entitled “Method of Manufacturing A Porous Material,” is hereby incorporated by reference, and discloses a manufacturing method of porous material. The method is essential for the laminar control method in U.S. Pat. No. 5,884,871, but it is not a method to control laminar flow.
U.S. Pat. No. 4,802,642, entitled “Control of Laminar Flow in Fluids by Means of Acoustic Energy,” is hereby incorporated by reference, and provides a method for laminar flow control by radiating acoustic energy into the boundary layer. The frequencies of the acoustics are greater than twice that of the critical Tollmein-Schlichting wave. Although the method is valid for subsonic flows, it is difficult to be applied to hypersonic flows. The method also requires a complicated system for acoustic generators and its control.
U.S. Pat. No. 4,664,345, entitled “Method for Stabilizing Laminar Separated Boundary Layers,” is hereby incorporated by reference, and provides a method to stabilize separated laminar boundary layers. The goal is achieved by suction and blowing inlets in the surface just upstream and downstream of the disturbance. This method is efficient for boundary layers experiencing a high pressure gradient. It is not valid for flow transition caused by instabilities.
For high Mach number flows (i.e. hypersonic flows), surface roughness has been previously used to trip boundary-layer transition by putting it near the leading edge of body surface. The height of surface roughness is generally higher than the local boundary-layer thickness. The stabilization effect of surface roughness has been mentioned in a couple of journal papers by experimentalists as failures of studies of how to trip transition efficiently when the height of roughness is lower than the local boundary-layer thickness. However, no explanation or further demonstration was reported. Surface roughness is currently still treated as a tool to trip hypersonic boundary-layer transition.
A review of the foregoing patents and applications shows there are quite limited methods for the control of hypersonic laminar flow of the boundary layer. Two methods in U.S. Patent Application Publication No. 2004/0195462 A1 and U.S. Pat. No. 5,884,871 are valid for hypersonic flows. However, the described techniques are complex and difficult to implement. There is a need in the art for techniques for controlling supersonic laminar flow of the boundary layer that are simple and convenient. There is a need for such techniques to be passive, depending only upon geometric parameters independent of material properties. In addition, there is a need for such techniques to efficient and flexible to design and manufacture without requiring changes to the fundamental vehicle structure. There is further a need for such techniques to be adjustable on a particular vehicle. These and other needs are met by embodiments of the present invention as detailed hereafter.